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Hassane Baghdad Kichou

Ph.D. Thesis title:
Aportaciones a los ensayos no destructivos por ondas de Lamb en aire 

Author:
Hassane Baghdad Kichou

Director:

Dr. Miguel J. García Hernández

Reading day:

12th July 2011 

 

Abstract:
This thesis develops a mathematical model and an algorithm for the full acoustic path of a system using Lamb waves, with application to nondestructive testing of solid sheets of mesh using air as coupling medium. This model lets to predict theoretically the spatial amplitude distribution of the acoustic field for each possible Lamb modes through the propagation sheet.
The ultrasonic nondestructive testing are in many cases the only essential tools for quality control in manufacturing products of high economic value, particularly when you need a very fast and accurate inspection of large structures, and the cost should be minimal.
Conventional methods of nondestructive testing require a coupling medium between the transducer and the test items. Mostly used coupling medium is a thin layer of liquid or gel, which decreases the efficiency and speed of inspection, especially when it comes to evaluating surfaces with irregularities. By automating this inspection is difficult and uncomfortable.
The work of this thesis agrees with the current research trends on the use of air as a coupling medium to improve the acoustic efficiency of ultrasonic transducers operating in air. The great advantage of using air is the removal of liquid materials and links, giving it the distinction of being a virtually non-contact inspection.
The central objective is to establish the optimal electro-acoustic transduction parameters for the improvement of NDT systems coupled by air normally using high frequency (or high resolution) piezoelectric transducers.

The following ones are considered outstanding contributions of this thesis 
A new direct numerical method development, without approximations and with very high resolution, easy to implement, requiring few computer resources and don¿t needing to use complex programs. It¿s the method to calculate the propagation characteristics for all possible Lamb modes dispersion curves in a homogeneous isotropic material, and the stress and strain generated by each mode in the material.
Direct calculation method development for modeling the acoustic field radiated by arbitrary profile, different layers apertures and structure by using the Rayleigh and Sommerfeld equations. The method, which fits any radiant aperture shape, including non-planar, accepts any type of apodization function and excitation shape. 
A collection of agile and easy to implement simplified models for the propagation of Lamb waves in laminated plates under continuous or pulsed wave excitation, with the introduction of geometry settings and acoustic field orientation of the transducers, the coupling medium, the material, thickness and the sheet angle. It gets the prediction of the spatial distribution of sound field of any Lamb mode, deviation and attenuation along the propagation path.
Introducing a new concept called the ¿admission (or transfer) diagram¿ to know the maximum energy transference angle for the generation of each Lamb wave mode that allows knowing the angle of incidence for maximum efficiency excitation associated with changes in the properties on the plate under test.
Modeling the change in amplitude of Lamb wave modes, caused by the alignment of the transducers to the surface, using air as coupling medium. We present an analytical model applicable to any mode of Lamb, which is able to determine the limits of an inspection system applied to sheets in movement.